INTEGRATED ROCK BED HEAT EXCHANGER-CUM-STORAGE UNIT FOR RESIDENTIAL-CUM-WATER HEATING. C. CHOUDHURY and H. P. GARGt

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1 INTEGRATED ROCK BED HEAT EXCHANGER-CUM-STORAGE UNIT FOR RESIDENTIAL-CUM-WATER HEATING C. CHOUDHURY and H. P. GARGt Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi , India (Received 28 October 1993; received for publication 8 December 1994) Abstract--This paper describes the results of a simulation study of a forced circulation, solar hybrid residential-cure-water heating system which comprises a corrugated absorber water heater and a rock-bed water-to-air heat exchanger-cure-storage unit integrated to a residential building to be heated. The system has been evaluated without and with the hot water load (which is the standard hot water demand for a typical family of two adults and two children in India) in the residential building. To satisfy the heat demand, no auxiliary energy has been assumed to be supplied to the system. The rate of air flow in the system was observed to influence the system performance most significantly. In addition, the mode of operation, i.e. whether the air flow is continuous or intermittent, influences the system performance greatly, a higher and intermittent air flow rate resulting in a more stable and uniform temperature of the living space. Rock bed.storage unit Heat exchanger Corrugated plate Solar collector to-air heat exchanger Solar room heater Domestic water heater Integrated water- NOMENCLATURE A = Surface area (m 2) B = Breadth (m) C = Specific heat capacity (Wh/kg C) d = Spacing between absorber and glass cover (m) D = Diameter (m) h = Heat transfer coefficient (W/m2 C) H = Height (m) 1 = Solar flux (W/m 2) L = Length (m) m = Mass (kg) = Specific mass flow rate of air (kg/h m 2) M = Mass (kg/m 2) A;I = Flow rate (kg/h) n = Number of windows S = Wall thickness of residential apartments (m) t = Time (s) T = Temperature ( C) U = Overall heat transfer coefficient (W/m2 C) W = Width (m) V = Volume (m 3) Z = Average duct depth in water heater (m) ~t = Solar ahsorptance r = Solar transmittance O = Angle of inclination of solar water heater Subscript 1 = Upper plate (glass cover) 2 = Middle plate (absorber) 3 = Inner plate (back plate) a = Heat exchanger air amb = Ambient air tto whom all correspondence should be addressed. tcm ~/10--E 999

2 1000 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER C = Solar collector D = Door e --- Enclosed air (in attic and closet) E = East wall i = Inlet lm = Logarithmic mean N = North wall o = Outlet r = Rock-bed R -- Room air s = Storage tank S = Steel w = Water W = West wall WI = Window INTRODUCTION The most important advantage of a hybrid heating system with a water-to-air heat exchanger-cumstorage unit is its multipurpose applications, like water heating, drying, space heating, space cooling (by letting cold water flow through the system) during different times of the year. A rock-bed heat exchanger-cum-storage unit has been used successfully for heat transfer (from water to air) and heat storage in the Thomason residential heating system in Washington, D.C., U.S.A. [1], Although the system was operated for several years thereafter, no indepth investigation on the design was conducted, which includes the parametric study of the system. In a recent communication[2], we have made detailed theoretical parametric investigations on an identical system which is confined to the study of the performance sensitivity to the system parameters when it is exploited for space heating applications only. This paper deals with the performance evaluation of the system with optimized design parameters when operated with and without the domestic hot water load on the system. The load is the standard hot water demand of a four person (two adults and two children) residential building in India. The system has been evaluated under both continuous and intermittent (i.e. air flow rate = 0 when TR > 27 C; here, 27 C has been assumed as the upper limit of the comfortable range for the residential building) air circulation conditions. To satisfy the space heating and the hot water demand, no auxiliary energy has been assumed to be supplied to the system. to Water ~ ~ ~ tank F:~ j i' '~..... "' '- --K,2 x,~ Rocks Fig. 1. The solar residential-cum-water heating system.

3 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER I001 /~'Wrlte r ulotion (a) Hot water / _ storage tank - - I /,/'" "", J Rock s ~ ~ ~ / f/ f Insulation (b) Fig. 2. Design details of: (a) corrugated absorber water heater; and (b) rock-bed water-to-air heat exchanger-cum-storage unit. DESIGN DETAILS The essential parts of the solar hybrid residential-cum-water heating system, i.e. (i) solar water heating collector, (ii) rock-bed water-to-air heat exchanger-cum-storage unit and (iii) residential apartment, are illustrated in Figs 1 and 2. The dimensional and operational details of the three units are summarised in Table 1. The water heating collectors on the south wall and the roof at 60 and 45 inclinations are duct type, corrugated absorber water heaters with a single glazing of glass, water flow between the corrugated absorber and rear plate with adequate insulation at the bottom and the sides. The water flows through the system only during the sunshine hours when T2 > Tws. Table I. Design and operational parameters of different components of the solar hybrid heating system Design parameters Water heater: Lc =4.8m at 9 =45 Lc=3.2m at (9 =60 Wc=8m Z = m d = 0.03 m Heat-exchanger-cure-heat-storage unit: Vw~ = 3.2 m 3 v~ = vws Lws/Dws = 1.2 Df = 0.03 m Residential apartment: HR=3m BR=8m WR=7m Awj = 1 m 2 Ao--- 2m 2 BN,E, w = 2 S =0.3m Operational parameter rh w = 100 kg/h m 2

4 1002 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER The heat exchanger-cum-storage is a cylindrical stainless steel water tank enclosed within an insulated rectangular tank, the annular space of which is filled with spherical rocks of size 0.03 m. The water tank is connected through insulated pipes to the inlet and outlet ducts of the water heaters, whereas the rock tank is connected to the inlets and exits of the living room and bed rooms of the residential flat. The flat is a two bed room set with kitchen, toilet/bath and living room which are separated from each other by short height thin walls to facilitate proper air circulation and, hence, uniform temperature throughout the space. The floor and the walls of the house are assumed to be made of bricks, whereas the roof is made of concrete. The solar water heater covers the whole of the south wall and part of the roof of the house. DOMESTIC HOT WATER LOAD Figure 3 shows a hot water draw profile which is the standard hot water demand for a family of two children and two adults during different times of a day in India. The temperature at which the hot water is used is 40 C which can be obtained by diluting, if necessary, the storage tank hot water with cool water at the supply temperature. The supply water temperature is assumed to be 15 C. Figure 3 also shows the total energy demand to be satisfied by a DHW system. OPERATING CONDITION In this water-cum-space heating system, the tank water is assumed to flow through the collector during the sunshine hours only, whereas the air flows through the apartment and the rock bed both continuously and intermittently (i.e. M = 0 when TR < 27 C, since a room temperature above 27 C is not considered comfortable). In addition, hot water from the storage tank is drawn for domestic use during morning, noon and evening hours as per the domestic hot water draw profile, and supply water at 15 C is added to fill the storage tank at the respective times of the day. It, deg. C J*lO 4 70 I Water Load (It) ~ Load Temg ( C) [~ Load Energy (J*104) 800 6O 7O O 3OO O AM-8 AM 12 N-1 PM 6 PM-7 PM Fig. 3. Domestic hot water load profile for a four person residential building.

5 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER 1003 THEORETICAL MODEL The energy balance equations for the different components of the water-cum-space heating systems are presented below. Solar water heater Ml C1 (dtl/dt) = oq I + h21 (T2 - Tl ) -- hl~r~b(tl - T~mb) M2C2(dT2/dt) = z,~2i - h21(t2 - Tt) - h2w(z2 -- Tw) M3C3(dT3/dt ) = hw3(tw -- T3) -- h3e(t 3 -- T~) ~l, Cw(dTw/dt) = -rhwcw(two - Two) + h2w(t2 - Tw) - hw3(tw - T3). (1) (2) (3) (4) Heat exchanger-cum-storage unit MC(dTws/dt) = rhw Cw(Two - Tws) - (LOAD)w - hs~(ts - Tf) - hwa AT, m (5) MrCr(dTr/dt) = hs~(ts - Tr) - hra(tr - Ta) (6) ~laca(dta/dt) = -/~ta C~(Tao - TR) + hwa ATom + hra(t, - Ta). (7) In equation (5), MC = Mw~Cw + M~C~ and ATIm is the logarithmic mean of AT~ and ATo, where ATi = T,,~- TR and ATo = Tw~- Tao, which, for small values of ATi and ATo, approaches the arithmatic mean of AT~ and ATo [3]. Residential apartment air mr CR(dTR/dt) = [(O~NIN/hNamb) "1- (Tam b -- TR)]URambA N "q-(ctele/heamb) "b (Tam b -- TR)]URambA E --I-(0~w/w/hwamb) --[- (Tam b -- TR)]URambA w +(ZIIEnEAwl + rllwnwawl + ZIINnNAWl)aR + l~laca(tao- TR). (8) RESULTS AND DISCUSSION The solar residential heating system was evaluated for a typical weather condition of Delhi in the month of January. Figure 4 exhibits the hourly values of the ambient temperature and the solar flux incident on the north, east and west walls of the house and on the south facing solar panels mounted on the roof and the south wall at 45 and 60 inclinations, respectively. The base values of the design and operational parameters for which the system is evaluated and summarized in Table 1. The results for different flow rates of air circulating continuously through the living apartment and the rockbed without the hot water load are presented in Fig. 5. Since larger air flow rates result in larger heat transfer from the water to the air, with increase in the air flow rate, the peak room temperature in the afternoon is observed to increase up to 35 C. However, a room air temperature above 27 C is not considered comfortable. To achieve a condition for comfortable room temperature, computations for intermittent air flow (i.e. M = 0 when TR > 27 C) without the hot water load were carried on for two consecutive days, the results of which are presented in Fig. 6. With the intermittent air flow, since the air flow is stopped frequently during the afternoon hours, heat transfer from the water to the air is stopped frequently, and hence, more energy is retained in the storage tank water which results in a higher temperature of the water than it is in the case of the continuous flow of air. Moreover, as larger air flow rates result in larger heat transfer from the water to the air, with an increase in the air flow rate, the temperature of the storage tank water decreases, whereas that of the apartment air increases.

6 1004 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER 1100 FLUX (W/sq.m) TEMPERATURE (*C) '-)g" 1(45") "-I-- I(SO") ~ I(E) -B- I(W) --X-- I(N) ~ Tamb Fig. 4. Hourly variations of ambient temperature ( T a m b ) and solar flux incident on north, west and east walls and on solar panels (SP) mounted at 45 and 60 on roof and south wall, respectively. Similar results with the hot water load on the system are presented in Fig. 7. Here, due to frequent extraction of hot water from the storage tank on the second day of operation, the water temperature is observed to be slightly lower than that in the case without the load in the system TEMPERATURE (deg.c) "q-- I~la-lO0 kg/h I~la-300 kg/h --)g-- I~la=600 kg/h 65 - ~ - ~ - - z : =! I I I I I I I I I I I Fig. 5. Hourly variations of apartment air (---) and storage tank water ( ) temperatures for different rates of continuous air flow without hot water load.

7 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER 1005 TEMPERATURE (deg.c) 105 / ~ Ivla=lO0 kg/h ~ ~,la=300 kg/h l 5 i I I I I I I Fig. 6. Hourly variations of apartment air (- - -) and storage tank water ( ) temperatures for different rates of intermittent air flow (i.e. ~t,~ = 0 if T R ~> 27 C) without hot water load. The temperatures for intermittent air flow as obtained for 4 consecutive days, assuming a hypothetical condition that the same climatological condition prevails during all 4 days, (with the hot water load from the 2nd day onwards) are presented in Fig. 8. As the number of days of operation increases, the difference between the maximum and the minimum of the water and the TEMPERATURE {deg.c) 105 Ma-lO0 kg/h ~ Ma-300 kg/h 95 -E}- IVla=500 kg/h ~ I~a=700 kg/h ~ 5 i i i i i i i Fig. 7. Hourly variations of apartment air (---) and storage tank water ( ) temperatures for different rates of intermittent air flow (i.e..~/'~ = 0 if T R >/27 C) with hot water load on second day.

8 1006 CHOUDHURY and GARG: INTEGRATED ROCK BED HEAT EXCHANGER TEMPERATURE (dog.c) st day --I-- 2nd day --Y~ 3rd day -B- 4th day ~ Fig. 8. Hourly variations of apartment air (---) and storage tank water ( ) temperatures for intermittent air flow (i.e. ~r a = 0 if T R 1> 27 C) for 4 consecutive days with hot water load (from 2nd day onwards) at K/" a = 500 kg/h. room temperature decreases. On the 4th day of operation, the room temperature is observed to vary between 22 and 27 C (which is very much in the comfortable range), whereas the water temperature varies between 65 and 90 C. This high value of the water temperature clearly shows the potential of the system for domestic hot water production for more than four persons, as well, in Indian weather conditions without the supply of auxiliary energy to the system. CONCLUSIONS The major accomplishment in this study is the performance evaluation of the solar space heating system without and with the hot water load for a four person residential building without integrating any supplementary heat source to the system. The rate of air flow and the mode of operation (i.e. whether air flow is continuous or intermittent) are observed to influence the system performance greatly. At the higher air flow rate and at the intermittent air flow condition, the system shows potential for fulfilling the residential space and water heating demand for a building of more than four persons without the use of auxiliary energy. REFERENCES I. H. E. Thomason, Solar Energy 10, 17 (1960). 2. C. Choudhury and H. P. Garg, Renewable Energy 2, 363 (1992). 3. D. Q. Kern, Process Heat Transfer. McGraw-Hill, New York (1986).